Pipe unions have long been employed to couple together axially aligned sections of pipe, conduit, or similar tubular flow lines. Buried lengths of such pipe commonly experience electrolytic corrosion, which substantially decreases the life of the flow lines. In order to reduce or eliminate such corrosion, insulated pipe unions have been employed, which electrically isolate the coupled union sections and therefore the lengths of pipe in order to prevent current flow through the system. Such insulated unions typically employ a plastic laminate bonded to the tail sub, so that the threaded sub and nut engage the electrically insulated plastic layer rather than the metal tail sub.
Both insulated and non-insulated unions typically employ a groove in the threaded sub for an O-ring. When the union is made up, the O-ring provides the seal between the threaded sub and tail sub (in the case of an insulated union, this seal is more precisely between the threaded sub and the plastic laminate) to prevent loss of fluid from the union.
Although the above described insulating unions have been employed in many applications, such unions experience a problem when used in gas flow lines operating under relatively high pressures, i.e., in excess of 1000 psi. As previously indicated, the plastic layer is ideally bonded to the end of the tail sub during the molding operation. In many instances, however, a gas-tight bond is not maintained over a long period of time, and leaks develop between the tail sub and the plastic layer in high pressure gas insulations. A gas leak in such an insulating union can be very expensive to repair, often requiring a complete shutdown of the system. Also, it is irrelevant from a practical standpoint that 99% of the unions may not leak at any point in time, since a system with one leak will still have to be repaired, and since after repair, another leak may develop. Moreover, the repeated making up and breaking apart of unions during service operations frequently causes prior art unions to leak even at relatively low pressure, e.g., 200 psi. Finally, gas leakage from any portion of a pressurized system may be extremely dangerous, as unfortunately evidenced by nationwide explosion reports. Accordingly, extremely high quality and repeatedly effective insulating unions are desired.
In an attempt to reduce leakage of gas from an insulating union, several attempts have been made to utilize special bonding agents and/or adhesives, either before or after molding the laminate over the tail sub. Although these solutions may decrease the likelihood of a leak, such attempts have not been 100% effective, and some unions still leak. Moreover, these attempts are labor intensive, and significantly increase the cost of manufacturing the insulating union.
The disadvantages of the prior art are overcome by the present invention, and an improved insulating union and method of manufacturing an improved insulating union are hereinafter disclosed.